Documentation: Add design rationale to contributing guide

[elskwid]

Add any design rationale for standout differences between this (Ruby) and other libraries or the spec.

From @fbogsany

I've been thinking recently about the first one, since we have some distinct Ruby-isms in our codebase that differ significantly from other implementations.

[benedictfischer09]

I would like to work on this

[mwear]

Thanks. It's assigned @benedictfischer09!

[plantfansam]

Is this issue still relevant?

[github-actions]

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